fix: remove @antora/pdf-extension to unblock CI (#3140)

The @antora/pdf-extension requires the asciidoctor-pdf Ruby gem to
convert assembled AsciiDoc to PDF. This gem was never installed in the
che-docs container, so PDF generation silently failed. After the base
image (quay.io/podman/stable:latest) updated, the extension started
crashing with ENOENT instead of failing silently:

  FATAL (antora): ENOENT: no such file or directory, open
  'build/assembler-pdf/docs/next/_exports/discover.pdf'

This breaks all PR checks. Since PDFs are not served on the live site
and the extension comment confirms it was used for "monolithic AsciiDoc
files" (not actual PDFs), removing it is safe.

The @antora/assembler and @antora/pdf-extension npm packages remain in
the Containerfile for potential future use.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: gtrivedi88

antora-playbook-for-development.yml

@@ -20,7 +20,6 @@ antora:
             snippet_length: 142
         -   require: '@antora/collector-extension' # Single-source content from engineering repositories, see https://gitlab.com/antora/antora-collector-extension
         -   require: ./extensions/htmltest.js # Test links in HTML, see https://docs.antora.org/antora/latest/extend/extension-tutorial/
-        -   require: '@antora/pdf-extension' # Generate monolithic AsciiDoc files, see  https://gitlab.com/antora/antora-assembler
 
 asciidoc:
     sourcemap: true